1. Field of the Invention
This invention relates to a process for preparing a mesophase pitch which has a low softening point and is homogeneous. More specifically, this invention is concerned with a process for preparing a mesophase pitch from a high melecular weight bituminous material obtained from a heavy oil of petroleum or coal origin, by hydrogenation thereof under heating in the presence of a hydrogen-donating solvent, and a successive heat treatment of the thus hydrogenated bituminous material, and is particularly directed to a process for preparing a mesophase pitch characterized in that the high molecular weight bituminous material is produced through the following three steps: the preliminary step of producing a refined heavy oil or heavy component which comprises adding a monocyclic aromatic hydrocarbon solvent to a heavy oil of petroleum or coal origin or a heavy component obtainable by a distillation, a heat treatment or a hydro-treatment thereof, separating and removing the insoluble components; the first step of subjecting the refined heavy oil or heavy component to a heat treatment in a tubular heater in the presence or absence of an aromatic oil; and the second step of adding a monocyclic aromatic hydrocarbon solvent to the thus heat-treated material and recovering the insoluble component newly formed in the first step. In some cases, the preliminary step can be omitted. The mesophase pitch prepared by the process of this invention is particularly suitable as a spinning pitch for producing high performance carbon fibers.
The high performance carbon fiber is light in weight, and has a high strength and a high modulus of elasticity, and therefore, the substance is highly valuable as composite materials usable for various parts of aircrafts, sports goods, industrial robots, and the like. Demands of the high performance carbon fiber are expected to largely increase in future.
2. Description of the Prior Arts
Hitherto, a major source of the high performance carbon fiber has been polyacrylonitrile (PAN) based carbon fibers which are produced by spinning PAN, rendering them infusible in an oxydizing atmosphere, and carbonizing or graphitizing them in an inert gas atmosphere. In recent years, however, processes were found to produce from pitches high performance carbon fibers which are competitive or even superior to the PAN based carbon fibers in their properties. Since pitches are an inexpensive raw material, the findings have drawn a great attention as a route for preparing high performance carbon fibers at a low cost.
In preparing the high performance carbon fibers from a pitch, the spinning pitch must be a so-called mesophase pitch which contains, as a major component, the substance exhibiting an optically anisotropic mesophase when examined on a polarized microscope.
This mesophase is a kind of liquid crystals which is formed when a heavy oil or a pitch is thermally treated, and its optically anisotropic character is due to an agglomerated layered structure of thermally polymerized planar aromatic molecules. When such mesophase is subjected to melt spinning, the planar aromatic molecules are aligned to the direction of the fiber axis due to the stress exerted to the melt as it passes through a nozzle hole, and this oriented structure can be kept without being disrupted throughout subsequent steps to render it infusible and carbonization steps, and therefore, high performance carbon fibers having good orientation can be obtained. On the contrary, when an isotropic pitch containing no mesophase is used, such orientation does not occur sufficiently by the stress when molten pitch passes through a nozzle hole because of the insufficient development of planar structure of molecules, and this renders the fibers poorly oriented and produces a carbon fiber with a lower strength, even if it is rendered infusible and carbonized. Therefore, a number of known processes for the manufacture of a high performance carbon fiber from pitches are directed to the method for preparing mesophase pitches spinnable into the fiber.
In the decade of 1965-1974, the mesophase was considered as equivalent of the substance insoluble in polar solvents such as quinoline and pyridine because of the fact that the mesophase produced by the thermal treatment was insoluble in such polar solvents. Subsequent studies on the mesophase, however, have unveiled the fact that the portion of the pitch which exhibits anisotropy under a polarized microscope is not necessarily the same substances with polar solvent insoluble substances, and further that the mesophase is composed of both polar solvent soluble and insoluble components. It is thus common nowadays to define the term "mesophase" as "a portion exhibiting optical anisotropy when examined on a polarized microscope". Furthermore, it is general to express the mesophase content by the ratio of areas exhibiting optical anisotropy and isotropy when a pitch is examined on a polarized microscope.
The mesophase content as determined according to this definition represents a property of a pitch having a great significance on its spinnability as well as the characteristics of the carbon fiber made therefrom. Japanese Patent Laid-Open No. 55625/1979 describes a pitch containing essentially 100% of mesophase, and states that it is desirable to reduce an isotropic portion as much as possible, because the presence of isotropic portion interferes with the spinning operation. The reason is that a pitch with a smaller mesophase content tends to separate into two phases in a molten state due to the lower viscosity of the isotropic portion than the anisotropic mesophase. When on tries, however, to increase the mesophase content of a pitch, the softening point and the viscosity become significantly high, making it difficult to spin the pitch. Thus, the most important problem in a process for preparing a high performance carbon fiber from a mesophase pitch resides in the fact that a significantly high temperature is necessary to use at the spinning stage because of the high softening point of the pitch. Spinning at a temperature of above 350.degree. C. involves such problems as cutting off of fibers and decrease of the fiber strength resulting from decomposition, deterioration, or thermal polymerization of the pitch in the spinning facility. Since a temperature which is 20.degree.-40.degree. C. higher than the Mettler method softening point of the pitch is generally required for the spinning, the softening point of the mesophase pitch must be below 320.degree. C. in order to keep the spinning temperature lower than 350.degree. C. The pitches described in Japanese Patent Laid-Open No. 55625/1979 have Mettler method softening point of 330.degree.-350.degree. C., which is not necessarily sufficiently low for the spinning operation, and in the examples, spinning is carried out at a high temperature of above 350.degree. C.
Japanese Patent Laid-open No. 154792/1983 discloses a quinoline soluble mesophase, and states that the content of the quinoline soluble mesophase in a pitch must be higher than a specific amount because the quinoline or pyridine insoluble mesophase raises the softening point of a mesophase pitch. There is no detailed description in this Japanese Patent Laid-open about the differences between the quinoline insoluble and soluble mesophase, but it may easily be understood that a highly polymerized substance with an extraordinarily high molecular weight would be insoluble in quinoline, and therefore, in other words, an attempt for preparing a pitch with a high quinoline soluble content would lead to an effort to reduce the content of such extraordinarily high molecular weight components and to prepare a homogeneous pitch having a narrow molecular weight distribution.
It is easy to reduce the quinoline insoluble component itself by, for example, employing a mild heat treating condition. But, this leads to a significant decrease in the mesophase content and an increase in low molecular weight components which are soluble in a solvent such as xylene. This xylene soluble low molecular weight component will have an adverse effect to the orientation of the fiber while spinning, and evaporate at the spinning temperature giving a cause of the fiber cut off. Therefore, in order to prepare a mesophase pitch with an excellent quality, it is not sufficient merely to decrease the content of exceedingly high molecular weight components which are insoluble in quinoline. Xylene soluble low molecular weight components must also be decreased, so as to make the pitch homogeneous and increase the content of intermediate components.
Various methods have been proposed other than those described above for preparing such homogeneous pitches. In one of the methods, an isotropic pitch is extracted by a solvent and the insoluble components are thermally treated at a temperature of 230.degree.-400.degree. C. (Japanese Patent Laid-opn No. 160427/1979). Other methods comprise hydrogenation of an isotropic pitch in the presence of a hydrogen-donating solvent, followed by a heat treatment (Japanese Patent Laid-open Nos. 214531/1983 and 196292/1983). Still other method employs a repetition of a thermal treatment on a pitch which was obtained by removing mesophase from a thermally treated isotropic pitch (Japanese Patent Laid-open No. 136835/1983). Further, still other method can give a pitch containing 20-80% of mesophase by a thermal treatment, and then recover the mesophase by precipitation (Japanese Patent laid-open No. 119984/1982). The pitches prepared by these methods, however, are not necessarily satisfactory, i.e., some pitches have a sufficiently high mesophase content but not sufficiently low softening point, some have a sufficiently low softening point but do not have a sufficiently high mesophase content, some pitches have both a low softening point and a high mesophase content but contains a large amount of significantly high molecular weight mesophase which is insoluble in quinoline and the like and cannot be deemed as homogeneous pitch. We have proposed processes for preparing pitches for use in the production of carbon fibers (Japanese Patent Laid-open Nos. 103989/1986 and 238885/1986), but the pitches obtained from these processes cannot be deemed as satisfactorily excellent pitches, yet. None of these methods is successful in providing a pitch satisfying the following four requisite properties at the same time, that is: (1) a low softening point, (2) a high mesophase content, (3) a low quinoline insoluble content, and (4) a low xylene soluble contenent.